In the new third edition rules, you take the level of the caster plus the level of the spell. That is lets say an 8th level caster and a 6st level spell. That is 14. You saving throw is your bonuses and your roll on the 1d20 (also called a twenty sider) need to be higher than, in this case, 14 to make the save vs spell.
Some times the save is for half, and some spells have no saves.
the 2nd edition rules that BGII is based on is similar. You have a save table by class with racial and equipement bonuses. If you roll higher than it, you save, but there is also the half or the no saving. Some spells also put a -4 or so on your saves for an adjustment for level, ie if you are a 4th level ranger and you are trying to save vs a 20th level mage, you should have some sort of adjustment for it...
BK